1. Field of the Invention
The invention relates to a process for the production of polymethacrylate molding compounds having high heat deflection temperature and high stability against thermal degradation, whereby at least 70% of the molding compound is produced with a monomer concentration of more than 5 mol methacrylate per liter of reaction mixture.
2. Description of the Background
Polyethacrylate molding compounds are usually produced by discontinuous or continuous bulk polymerization and by bead polymerization. The discontinuous process, performed at low temperatures, e.g. between 20.degree. and 50.degree. C. is very time and work intensive. For this reason it has more recently been almost completely replaced with continuous, but technologically complex polymerization processes.
The continuous methacrylate polymerization is performed e.g. in a zone of rising temperatures from 130.degree. to 250.degree. C. in a polymerization unit constructed as an extruder. Prior to the isolation of the polymer as an extrudate, unreacted monomer having a content of up to 40 wt. % of the batch monomer, is removed in a degasification zone and is returned to the polymerization zone. During polymerization, high space-time yields can be attained only by performing the polymerization at temperatures of approximately 100.degree. C. and in the presence of relatively high initiator concentrations (cf. e.g. Vieweg, Esser, Kunststoff-Handbuch, Vol. IX, pp. 22 to 35, Carl Hanser Verlag, Munich, 1975, or Ullmanns Enzyklopadie der technischen Chemie, 4th ed., Vol. 19, pp. 22-27, Verlag Chemie, Weinheim, 1980). Also known is the production of polymethacrylate molding compounds by solvent polymerization and subsequent evaporation of the solvent.
To stabilize against the thermal degradation of polymethacrylate molding compounds, the methacrylate is generally copolymerized with a few percentages of an acrylate or with styrene.
The polymerization processes for producing polymethacrylate molding compounds, which can be processed thermoplastically, are generally performed in the presence of chain transfer agents, particularly in the presence of mercaptans which interfere with the polymerization mechanisms as chain stoppers and thus influence the degree of polymerization and the chemical structure of the polymer end groups. Using mercaptans as the best known chain transfer agents, this generally results in polymers having a higher number of thermally more stable end groups than polymethacrylates produced without chain transfer agents (DE-PS 16 45 232). The nature of the end group is of particular significance for the thermal stability of polymethacrylate molding compounds. It was e.g. possible to demonstrate in model experiments that polymethacrylate polymer chains with saturated end groups, such as are formed by transfer, are stable up to temperatures above 300.degree. C., while polymer chains formed by disproportionation termination, and thus have an olefinic double bond at the chain end, already undergo thermal degradation at approximately 250.degree. C. [P. Cacioli et al., Polymer Bulletin 11, 325 (1984)]. Particularly unstable are polymethylmethacrylate (PMMA) polymer chains produced by recombination termination; these are thermally stable only up to a temperature of 190.degree. C.
To attain a satisfactory thermal stabilization, it is advantageous that the incorporation of acrylates and the polymerization in the presence of mercaptans are combined in the technical production processes of polymethacrylate molding compounds. Further, additional low molecular stabilizers are added in order to improve processability.
The heat deflection temperature of polymethacrylate molding compounds, which may be defined e.g. by the glass transition temperature (T.sub.g) or the Vicat softening temperature VST according to DIN 53460, is influenced significantly by the tacticity of the polymethacrylate polymer chains which can be varied widely depending on the polymerization process. A high content of syndiotactic and heterotactic triads, which results in molding compounds with a high heat deflection temperature, in addition to a low content of isotactic triads in the polymer chains, are technically desirable. Such tacticities are attained with low polymerization temperatures. Relatively high initiator concentrations must be used in such polymerization processes in order to attain high space-time yields. This results to a greater degree in polymer chains which, as already explained, are terminated by termination due to recombination termination or disproportionation termination and thus assumedly result in lower thermostability. On the other hand, polymerization at higher temperatures, e.g. in a continuous polymerization process at approximately 180.degree. C., results in polymers with good stability against thermal degradation in a good space-time yield using only a relatively small amount of initiator. Because of the high polymerization temperature, the higher temperature polymerization produces however, as a result of the high isotactic triad content, a polymer with a low heat deflection temperature. EP-PS 0 245 647 (=U.S. Pat. No. 4,877,853) provides a technical concept for overcoming this dilemma. Here a thermoplastically processable polymethacrylate molding compound with high heat deflection temperature and high stability against thermal degradation is described. The polymethacrylate has a content of more than 60% syndiotactic triads, and preferably an initiator-chain transfer agent ratio of less than 1:2. The polymethacrylate molding compound is produced at temperatures between 0.degree. and 100.degree. C. by emulsion polymerization.
Continuous polymerization processes performed with lower yields and high polymerization temperatures with good space-time yields are disclosed e.g. in Japanese Kokai Tokyo Koho JP 04,146,903, which describes the polymerization of methylmethacrylate (MMA) at yields up to 20 to 50%. The subsequent monomer degasification, as well as the final polymerization of the polymer syrup into PMMA results in polymer plates with good optical properties.
U.S. Pat. No. 2,974,125 discloses the production of copolymers from MMA, styrene and acrylonitrile by a polymerization which is performed at temperatures between 130.degree. and 155.degree. C. and with yields up to 10 to 50%. The unreacted monomers preferably are removed by degasification, and the resulting polymers are characterized by high strain at break values. DE-AS 25 04 417 describes a process for producing methylmethacrylate polymers containing at least 80 wt. % MMA units. In this process the monomers are reacted at temperatures between 150.degree. and 180.degree. C. until reaching a conversion of 50 to 80% in the stirred tank reactor, and the unreacted monomers are removed by applying a vacuum. The resulting polymers are free of residual monomers and oligomers up to a molecular weight of 1,000 Dalton.
U.S. Pat. No. 4,711,938 discloses a continuous process for producing PMMA or MMA-containing copolymers at polymerization temperatures from 140.degree. to 170.degree. C. and conversions of up to 50%. Residual monomers are also removed by the application of a vacuum. The resulting polymers are characterized by high optical purity and narrow molecular weight distribution. U.S. Pat. No. 3,637,545 describes the production of polymethylmethacrylate by way of continuous mass polymerization at temperatures between 145.degree. and 165.degree. C. and conversions between 40 and 60%.
The residual monomers were again removed by degasification. According to another embodiment of the invention elastomer particles may be present during the reaction.
EP Patent 0 319 622 B1 describes a process for the continuous solvent polymerization of (meth)acrylates by (a) adding monomers, polymerization aids and solvents in such a way that the polymer content of the reaction mixture remains below 50 wt. %, (b) conducting the reaction at temperatures of 60.degree. to 130.degree. C. and adequate residence times to ensure a 20 to 95% conversion of the monomers, (c) transferring the polymer into a different polymerization reactor for (d) subsequently polymerizing at reaction temperatures between 60.degree. and 130.degree. C., (e) transferring of the raw polymer produced in step (d) to a final polymerization reactor where the non-polymerized components of the reaction mixture are removed by flash degasification, and (f) specially finishing the final polymer.
Except for the concept described in EP-PS 0 245 647, the applications or patents described above do not offer an approach for solving the problem of how PMMA molding compounds can be produced which possess both a high heat deflection temperature, and thus a low content of isotactic triads, as well as a high stability against thermal degradation due to special end groups in the polymer chains. EP-PS 0 245 647 solves the problem of producing PMMA molding compounds with high heat deflection temperature and high stability against thermal degradation excellently. Since the molding compounds described in the patent are produced by emulsion polymerization, the isolation of such molding compounds from the aqueous dispersion is technically complex. A need, therefore, continues to exist for a technically less complex method of producing polymethacrylate molding compounds having high heat deflection temperature and high stability against thermal degradation.